if you're using nagios, setup something like command_name check_certificate command_line $USER1$/check_http -H $ARG1$ -p $ARG2$ -4 -S -C 21 -t 20 --sni this won't warn about certificate name mismatches though
That warns only after the fact, not in advance... depending on how the infrastructure (and especially the cert procuring process, for those not using LE for whatever reason) is set up, too late.
-C 21 means: Check the certificate. Warn if less than 21 days and critical when expired.